Recently, from the standpoint of global conservation, it is a worldwide problem to suppress the total amount of carbon dioxide (CO2) exhausted from an internal combustion engine such as automobile engine and the generated amount of nitrogen oxide (NOx). In order to solve this problem, a lean-burn engine has been developed for the purpose of improving the fuel consumption and an absorption reduction-type NOx purifying catalyst, obtained by adding a function of occluding NOx in a lean atmosphere to a conventional three-way catalyst, has been developed for the purpose of purifying the exhaust gas of the lean-burn engine. These technologies are having some success against the above-described problem.
In the lean-burn engine combined with the absorption reduction-type NOx, purifying catalyst, the fuel is usually burned in a lean (oxygen-excess) condition and temporarily burned in a stoichiometric (at a theoretical air-fuel ratio) or rich (fuel-excess) condition.
HC (hydrocarbon) or CO in the exhaust gas is efficiently burned and removed in the lean condition by the action of catalyst using the oxidative atmosphere. On the other hand, NOx is captured by an absorbent in the lean condition and this is temporarily released in the stoichiometric or rich condition and reduced and purified by the action of catalyst using the reducing atmosphere.
By virtue of these combustion conditions and actions of the absorption reduction-type NOx purifying catalyst, as a whole, the fuel consumption is improved and, at the same time, HC, CO and NOx in the exhaust gas can be purified with good efficiency.
In this absorption reduction-type NOx purifying catalyst, a noble metal such as Pt, Au, Pd and Rh is used as the catalyst component and an alkali metal such as K and Na or an alkaline earth metal such as Ca and Ba is used as the NOx absorbent.
This lean-burn system established by combining the control of an air-fuel ratio and the NOx absorbent is successful to a certain extent in solving the problem to improve the fuel consumption and reduce the total generation amount of CO, HC and NOx as compared with the conventional system using a three-way catalyst and combustion in the vicinity of theoretical air-fuel ratio.
Japanese Unexamined Patent Publication (Kokai) No. 9-24274 describes an exhaust gas purifying catalyst comprising a composite oxide containing alumina and zirconia and having an acid point and a base point, and Kokai No. 11-226404 describes a stabilized zirconia support for improving the steam reforming reactivity of catalyst component Rh.
However, the absorption reduction-type NOx purifying catalyst using the above-described alkali metal or alkaline earth metal as the NOx absorbent has a problem that the NOx purifying capability decreases when the exhaust gas temperature is high and exceeds about 500° C.
This is thought to occur because, although the alkali metal or alkaline earth metal exerts the function as NOx absorbent by absorbing NOx while forming a nitrate in the lean side and by releasing NOx while forming a carbonate in the rich side, the nitrate or the like of the alkali metal or alkaline earth metal decomposes at a high temperature exceeding about 500° C. and, therefore, the intended function cannot be brought out.
Accordingly, an object of the present invention is to provide an exhaust gas purifying catalyst capable of exerting high NOx purifying capability even at a high temperature exceeding about 500° C. by using, as the NOx absorbent, a specific modified ZrO2 composite oxide, different from those in conventional techniques, in place of an alkali metal or an alkaline earth metal.